There is known a technique referred to as remote copy, which transfers data in a volume being the copy source to a storage apparatus located at a remote site as the copy destination, such remote copy being used for disaster countermeasure, for example. In addition, there is a technique referred to as order-guaranteed remote copy, which copies data to the copy destination volume while maintaining the order of data update in the copy source volume.
The order-guaranteed remote copy stores, in the copy source buffer, data whose copy source volume has been updated, and transfers data from the copy source buffer to the copy destination volume in the order of updates. On this occasion, updating the copy source volume may cause exhaustion of copy source buffers when communication between storage apparatuses is disabled by network failure or the like.
The copy source storage apparatus creates a bit string corresponding to the update area in the copy source volume, from data stored in the copy source buffer, and subsequently uses the bit string to manage updates performed on the copy source volume until normal operation is resumed. The bit string is information on whether an update has been made for each predetermined area in the copy source volume, and does not include information relating to the order of updates. The remote copy therefore turns into an order-unguaranteed state until normal operation is resumed.
There have been attempts to suppress exhaustion of copy source buffers to prevent falling into an order-unguaranteed state.
Japanese Laid-open Patent Publication No. 2007-140651
Japanese Laid-open Patent Publication No. 2011-150602
Japanese Laid-open Patent Publication No. 2007-18236
However, once exhaustion of copy source buffers occurs, a bit string corresponding to the update area in the copy source volume is created and the copy source buffer is cleared. Data to be transferred corresponding to the bit string which has been created in the above manner grows to a size corresponding to a size of the copy source buffer and takes a long time to be transferred.
With such remote copy, the order-unguaranteed state continues for a long time due to exhaustion of copy source buffers and therefore it is desired to resume to the order-guaranteed state as soon as possible.